Can't create new category (3 posts)

When I write a new post or try to edit an older one, I cannot create a new category to post under. I've tried also going to manage categories, but the same thing happens. I type in the name of the new category, click create, and the name dissapears, but the category isn't created. I've tried shutting my browser down and even restaring my computer, but it didn't help. Anyone know if it's just me or is there something else going on?